Means for cutting external screw threads



Nov. 28, 1933. R'. THOMSON MEANS FOR CUTTING EXTERNAL SCREW THREADS sShets-Sheet 1 Original Filed April 7, 1950 l'moenfof ROBERT THOMSON(lttornegs Nov. 28, 1933.

Original Filed April 7,1930

3 Sheets-Sheet 2 ROBERT THOMSON E 5 f 2 I R R N liilililiiliiiliy' NOV.28, 1933. R THOMSON 1,936,696

MEANS FOR CUTTING EXTERNAL SCREW THREADS Original Filed April '7, 1930 3Sheets-Sheet 3 wig / lnvcntor ROBERT THOMSON mm \Hflll N Ottornq;

\ zzt Patented Nov. 28, 1933 MEANS FOR CUTTING EXTERNAL soasw massesRobert Thomson, Arlington, N. J., assignor to Dardelet ThreadlockCorporation, New York,

N. Y., a corporation of Delaware Continuation of applications Serial No.415,742, December 21, 1929, and Serial No. 442,224, April 7, 1930. Thisapplication May 6, 1932.

23 Claims.

This invention relates to improvements in means for cutting externalscrew threads, the improvements being especially applicable to cuttingsuch threads by means of a screw threading die, and being, especiallyuseful in cutting external threads of the kind known as Dardeletself-locking screw threads.

Important objects of the presentinvention are to provide improved meansfor cutting external screw threads (including Dardelet self-lockingthreads) whereby the out threads will be true as to pitch and profile,will not be cut "out of round, and may be cut with ease, expedition andeconomy without sacrificing accuracy of formation. Other importantobjects of the invention are to provide improved means (includingimproved die chasers or cutter.elements)= capable oi embodiment invarious kriown types of dies for use in die-cutting external threads(including Dardelet self-locking threads) with accuracy, speed andeconomy without the setting up of injurious relative radial, axial, orboth radial and axial, vibrations between the die and the piece beingthreaded.

The invention also includes among its purpose the provision of a threadchasing die for chasing or cutting a complete external Dardeletselilocking thread at a single operation without cutting the thread outof round (1. e. without cutting a thread varying in radial displacementfrom the thread axis as it winds about said axis) and without cuttingthe thread either out 01' pitch or with a distorted or varying profile.

The invention also includes among its pur-- poses the provision of a diefor completely cutting an external Dardelet thread which has a finishedor perfect profileup to a point within one turn of the inner end of thecut made by the die in the threaded piece; the provision of means foraccurately cutting an external Dardelet thread on rough commercialpieces which vary somewhat from a nominal diameter; and the provision ofan eiiicient die for cutting an external Dardelet thread close up to a.shoulder on a shouldered piece.

In the accompanying drawings, wherein the invention is shown embodied inseveral forms of Serial mately on the line 1-1 of Fig. 2, showing a boltthread being chased by the chasers of a die which is further illustratedin Figs. 2, 4 and 5;

Fig. 2 is a fragmentary sectional view on the line 2-2 01' Fig. 1;

Fig. 3 is a fragmentary view, partly in longitudinal section, of a boltand nut provided with Dardelet sell-locking threads, the nut and boltthreads being shown in locked relation;

Fig. 4 is a view showing in side elevation a'wellknown type of die asimproved by the embodiment therein of one form of the invention; 7 I

Fig. 5 is a view of the leading or entering and oi the improved dieshown in Figs. 1, 2 and 4;

Fig.6 is a perspective view of one of the chasers of said die; a

Fig. 7. is a plan view 0! the grooved face 01 one of said Chasers;

Fig. 8 is a view showing the active or cutting ends of"the severalchasers of the set of four chasers in said die;

Fig. 9 is a view of the leading or entering end of an improvedconstruction of another wellchasers oi the die shown in Fig. 12, as itap- S pears when looking directly at the cutting face of the chaser;

Fig. 14 is a detail view showing the mounting of chasers in the dieillustrated in Fig. 12;

Figs. 15 and 16 are views similar to Figs. 1 and 8, respectively,showing a modification of the improved threading die disclosed in Figs.1, 2, 4., 5, 6, 7 and 8;

Fig. 17 is a view showing a modified construction of the improved set ofehasers disclosed in Fig. 16; and

Fig. 18 is a fragmentary sectional view on the same line as Figs. 1 and15, showing how the Chasers of the modified form of die illustrated inFig. 15 act on slightly oversize stock, the particular chaser shown inFig. 18 being the uppermost chaser appearing in Fig. 15. i

' In the commercial Dardelet system of screw threading, the external andinternal threads, as of a bolt and nut for example, have the forms shownin Fig. 3, and are of equal pitch. As

shown, the rib 20 of the bolt thread and the rib 21 of the nut threadhave steep side walls making an angle of 14 degrees with the perpendicular to the thread axis, said ribs being much narrower than thethread grooves. The bolt thread has a slightly conoidal root surface 22making an angle of six degrees with the thread axis, upon which rootsurface the correspondingly conoidal crest surface 23 of the nut threadbecomes tightly wedged under elastic tension when the nut thread rib isforced (by reaction of the work on the nut) from the deeper to theshallower side of the bolt thread groove into the position shown in Fig.3 wherein one side face of the nut thread abuts that side face of thebolt thread which faces away from the entering end of the bolt. Thecrest surface of the bolt thread and root surface of the nut thread lieparallel with the thread axis from edge to edge ofsaid surfaces.

.It will be noted that the conoidal root surface of the bolt thread isof greatest diameter along the edge of the bolt thread groove which isnearest the entering end of the'bolt or the reaction taking steep sideface of the bolt thread rib. It will also be noted that this conoidalsurface slopes inward toward the axis of the cut I ting circle of thedie in a direction opposite to the slope of the customary outwardlyflaring throat portions on the die chasers at the leading end or face ofthe die.

Prior to the present invention, in cutting Dardelet threads it has beenthe practice to provide the chasers with groove developing ribs profiledto correspond with the profile of the bolt thread groove and also toprovide the usual inclined throat at the leading edge of the chaser asin the production of chasers for cutting the ordinary V-thread and otherknown types of thread, with the result that the rib between the firsttwo grooves of the chaser has a crest surface which either in whole orin part slopes in a direction opposite to the direction of inclinationof the throat portion of the chaser. In die cutting threads with suchchasers, it has been found that in many cases the dies cut out of round,out drunken threads, cut threads out of pitch, and cut thread profileswherein the rib (at least during the first one or two turns of thethread) is narrower than it should be and the groove correspondinglywider than it should be. In using such dies there is a tendency to setup between the stock and die undesirable relative axial and radialvibratory movements resulting in distorted cutting of the thread. It hasbeen found that the present invention overcomes the defects set forthand that, by practicing the invention, accurate threading may beaccomplished expeditiously and economically,

According to the embodiment of the present in-- vention shown in Figs.1, 2 and 4 to 14, inclusive, the set of chasers for the die of whatevertype have their grooved faces provided with parallel thread grooves Grunning in the known manner, each chaser having the usual beveled throatsurface T adjacent and along the leading edge'thereof intersecting thegroove G located nearest the leading edge of the chaser, said groovesall extending to a common base line. The depth of the grooves 15preferably such that the root surfaces of the grooves lie entirelyoutside the periphery 'of the bolt shank the die is designed to thread,as is usual. The ribs have steep side walls the slope of whichcorresponds with the slope of the side walls of the bolt thread groove,the included angle in the case of the standard Dardelet thread beingtwenty-nine degrees, as

shown. A plurality of the ribs designated R provided which is identicalwith ribs R except that it is lower, its top being cut away to provide acrest surface C' parallel with the base line of the grooves G. Thiscrest surface may extend from steep side wall to steep side wall of ribR and in most cases does so on all chasers of a set, or all except onechaser of the set. In some cases, due to the angle-of surface T to thedie axis, the beveled surface T is continued upon a portion of rib R ofat least one chaser. It will be understood that two or more ribs of thetype of rib R may be provided in advance of ribs R, if desired. Thesurfaces T, C and C' are preferably produced by grinding operations onthe grooved face of the chaser.

The die shown in Figs. 1, 2, 4, 5, 6, 7 and 8 is of the type shown in U.S. Patents No. 409,208

- and No. 864,964, wherein the chasers 24 are barlike chasers arrangedtangentially to the cutting circle, and the grooves G and ribs R, Rextend parallel with the longitudinal side edges of the chasers, thechasers being canted in the wellknown manner so that said ribs andgrooves extend at an angle to the axial line of the bolt correspondingwith the helix angle of the thread to be cut, and said chasers beingrigidly clamped in the die head. Further description of the die head andmounting of the chasers is believed to be unnecessary.

In the improved chasers for the die just described, each chaser 24 alsohas a beveled or rearwardly relieved cutting face portion F at one endextending transversely of the chaser from the leading edge of the chaserto a point beyond the leading one of the ribs R, said relieved facebeing produced by grinding one end of the chaser for a portion of thewidth of the chaser in a plane inclined to the flat back .of the chaser,said inclined plane making an angle with the 1ongitudinal side edges ofthe chaser equal to the helix angle of the thread the chaser is profiledto cut, thus leaving a relatively projecting following portion L thegrooves and ribs of whichact as a lead screw. When a chaser, having itsactive end formed as described, is set in the usual position in the diehead, as shown in Figs. 1, 2 and 5, the serrated or toothed cutting edgeof face portion F extends parallel with the axisof the die and of thebolt or other member being threaded, and end portion L will projectbeyond the cutting line, thus improving both the cutting and leading inactions of the die on the bolt.

The provision whereby rib R is interposed between the profile flnishingribs R of each chaser and throat T of the chaser, and whereby face 1? isextended beyond the leading rib R, also give 1,o'ae,coe.

a wide cutting edge in advance of leader portion L, which is anadvantageous feature."

The die shown in Figs. 9, 10 and 11 is constructed as shown in U. 8.Patent No. 1,111,412, except that the inner faces of its chasers 25 areprofiled, as heretofore described, in accordance with one of theembodiments of the present invention. As shown herein, this die is ofthe radially arranged chaser bar type wherein the chasers. are in effectsegments of a nut, the grooves and ribs and beveled throat surfaceextending across the concave inner end faces of the chaser bars andforming an interrupted cutting thread.

The die shown in Figs. 12, 13 and 14 is constructed as shown in U. S.Patent No. 220,592, except that the ribbed faces of its chasers 26 areprofiled, as heretofore described, in accordance with one oftheembodiments of the present invention. As shown henin, the chasers 26of this die are of the recessedcylindrical chaser type wherein thechaser cylinders have-annular ribs and grooves parallel with the sideedges of the chasers, the chasers being mounted on the die. head withtheir axe: canted relatively to the axis of the die as indicated in Fig.14, and in said patent, so that the ribs and grooves extend back fromthe cutting faces 27 of the chasers at an angle to the die axiscorresponding with the helix angle of the thread to be cut: Cuttingfaces 27 are formed on the chasers in the usual way by recessing thechasers as shown.

By providing dies embodying the invention in the forms above described,it will be observed that about two turns of an unfinished thread groovewill be cut inwardly from the entering end of the bolt before the firstrib R of a chaser (the first rib whose profile corresponds with thefinished thread groove profile, and the first rib having a crest surfaceC sloping in the same direction as the root surface of the finishedthread) begins the generation of the conoidal root surface of thethread, and it will also be observed that throughout one of these tunisthe steep side walls of the thread groove will be partly developed bythe four ribs R of the four chasers, which latter ribs owing to theirflat crests C out the lastmentioned turn of the thread with a bottomsurface which extends parallel with the bolt axis widthwise of thethread groove. There is thus presented for finishing cutting by the fourfol= lowing ribs R of the set of chasers a substantially completedgroove having its side walls finished to nearly their full depth buthaving its bottom wall extending parallel to the bolt axislongitudinally of said axis, said bottom wall of the substantiallycompleted groove having no slope in the direction of that of the rootsurface of the completed thread.

It will thus be seen that the finishing cutting ribs or teeth R, even atthe entering end of the bolt, convert a cylindrical root surface intothe conoidal root surface of the Dardelet thread, following along in thegroove behind cutting ribs or teeth R which bring the grooveprogressively to the above substantially completed form in advance ofsaid finishing teeth. By this arrangement the undesirable conditionshereinbefore mentioned are avoided, and threads are cut with greateraccuracy. The ribs or teeth R with non-sloping crest surfaces C not onlypresent to the action of the finishing ribs or teeth a root surface andside surfaces of such character that undesirable relative endwisevibratory and/or creeping movements between the stock and die are notdeveloped; but the non-sloping crest surfaces of said ribs R straddlethe stock and bear firmly thereon at circumferentially spacedpointsalong lines parallel with the axis of the stock'and die and effectivelycenter and steady the stock, these features being especially desirableat the start of the threading of the bolt.

In the embodiment of the invention disclosed in Figs. 15, 16 and 18, thedie (including its set of four chasers and their arrangement in thediehead) is of the same construction as that shown in Figs. 1, 2,4, 5,6, 'Z and 8 and above described, except that the chasers are differentlyformed at their ribbed faces from their leading side edges to the firstor leading one of the ribs R having the profile of the thread groove ofthe Dardelet thread, i. e. the first rib with an inclined top C. Each ofthe four chasers 24*, 24, 24 and 24. of this modified form of die has abeveled throat surface T extending inward from its leading side edgetoward the leading one R." of a series of parallel thread ribs R-R"which extendlongitudi nally of the chaser parallel with the longitudinalside edges of the chaser. Throat surface '13 makes the same angle withthe flat back of the chaser and with the die axis, and is of the samewidth, on all four chasers of the die. Throat surface T on each chaserintersects the common base line B of the series of ribs R of the chasershort of the leading rib R" with which it is connected by a flat orplane surface I? coincident with the base line B and the flat bottoms ofthe grooves G between the chaser ribs.

The several series of ribs on the successive chasers of the set of fourchasers are progressively offset from the leading side edges of thechasers 110 in the usual manner so that the active or innermost cuttingends of the ribs will lie in a helical path to form a helical series ofcutting teeth when the chasers are assembled in the usual manner in thedie head H. Since the flat surfaces 1? are ex- 115 tended to the leadingribs R" from throat surfaces T' of equal width on all the chasers, thesesurfaces P progressively increase in width from chaser to chaser of theset, being of least width on the uppermost chaser shown in Fig. 16 andof great- 120 est width on the lowermost chaser shown in Fig. 18, inwhich view the chasers are shown in the order from top to bottom of theview in which they act to start the progressive development or cuttingof the thread profile to its final form.

The thread grooves G of the modified chasers and the ribs R are similarto those of the set of chasers 24, the flat bottoms of said groovesbeing all equidistant from the fiat backs of the chasers, and the ribs Rbeing all of the same height and profile, which profile is identicalwith that of the thread groove of the external Dardelet seif-ioclcingthread. Grooves G are all of the same width at the base, and all theribs R-R" are also of the same width at the base. The side walls of theseveral ribs diverge toward the base line or" the ribs and (in theconstruction shown for cutting standard Dardelet threads) each of saidwalls makes an angle of 14 degrees with the perpendicular to base lineB.

The leading rib R" of each chaser has a flat top wall or crest surface Clying parallel with the base line B of the series of ribs Rr-R|" of thatchaser and also parallel with surface P of the chaser. The height ofribs R" of the several chasers varies on different chasers of the set,rib R" of chaser 24* being lowest, and ribs 'R" of chasers 24 to 24being progressively higher. Rib R" of chaser 24 is preferably made verylow as compared with the depth of the thread groove of the 150 Dardeletthread to be cut, and rib R" of chaser 24 is of no greater height (andpreferably of slightly less height) than the height of ribs R at thelower side edges of the latter ribs, so that the inclined top edge ofthe leading one of the ribs R on chaser 24 will complete thedevelopment'of the Dardelet thread groove by taking a thin cutthroughout its width. The tops C of all ribs R of each chaser inclinefrom edge to edge of the ribs in planes at an angle of six degrees toboth the base line B of the ribs and the flat back of the chaser, eachof said ribs being lowest at the side edge thereof that is farthest fromthroat surface T and leading rib R of the chaser, so that the throat Tand tops of ribs R are of opposite inclination.

In the die last described, the active or innermost cutting end faces ofthe modified chasers each has a beveled or relieved cutting end faceportion F and a relatively projecting guide end portion or lead-offportion L, with face portion F extending inward from the leading sideedge of the chaser to a point beyond the leading one of the ribs R andmaking an angle with the longitudinal side edges of the chaser equal tothe helix angle of the thread the chaser is profiled to cut, so thatface portions F extend parallel with the die axis and the axis of thepiece being threaded, while portion L slightly projects beyond the lineof cut for more efficient action as a guide or lead screw, as indicatedin dotted lines in Fig. 15 and as shown and described in connection withthe die disclosed in Figs. 1, 2, 4, 5, 6, 7 and 8.

In operation of a die of the construction shown in Figs. 15, 16 and 18,the activeor inner ends of the beveled throat surfaces T of the chaserswill act (when necessary) as cutting and guiding edges to center andguide in an oversize piece and taper it down, as shown in Fig. 1 8.

The active or inner ends of surfaces P of the chasers will straddle andbear 'on the periphery of the piece being threaded to hold it trulycentered and also against vibration, if the piece is of proper nominalsize, as shown in Fig. 15; and, if the piece is oversize, said ends ofsurfaces P will (as shown in Fig. 18) additionally act as cutting edgesto bring the oversized piece to cylindrical form of proper nominaldiameter in advance of the cutting of any groove, or the starting of athread groove in the piece, so that the groove developing ribs of thedie chasers will always begin to act upon a true cylindrical surfaceextending parallel to the axes of the die and the piece being threaded.

The cutting ends of the ribs of the" ser es of leading ribs R" of thedie will bring the thread to approximately final form by deve'oping fromthe very outset a groove with two steep side walls united by a bottomwall which is parallel from edge to edge with the axes of the die andpiece, and the flat tops C of said ribs will serve to aid portions P inholding the piece centered in the die, while the steep side walls ofsaid ribs will serve to prevent relative axial vibrations orreciprocations between the die and piece from the very beginning of thedeveopment of the thread on the piece.

These features are highly advantageous and important, as they preventthe setting up of axial and radial vibrations between the die and thepiece being threaded at the beginning of the cut ting of the thread,when such vibrations would have the greatest effect upon the accuracy ofthe product. Said features are sepecially advantageous in connectionwith the cutting of external Dardelet threads the profile of which issuch as to tend to augment the deficiencies of the usual types of diesin the foregoing respects, which deficiencies result frequently incutting threads that are out of pitch, out of round, andimperfect inprofile, especially in the first few turns of the thread inward from theentering end of a bolt or other male piece.

Other highly desirable features of the invention embodied in themodified form of die disclosed in Figs. 15, 16 and 18 are that itprovides for cutting the thread to finished "and perfect profile towithin one turn of the end of the helical cut made in the piece by thedie, and delays the generation of the conoidal root surface of theDardelet thread until the groove is developed to approximately fulldepth with steep side walls and a broad bottom wall which is parallelwith the axes of the die and piece. These features permit cutting athread of full profile very close to the inner end of the cut made bythe die, and prevent the die from tending in operation to pull in thepiece being threaded more rapidly than called for by the thread pitchand nominal lead rate of the die and thread (especially at the start ofthe threading) in such manner as to cause the leading low height ribs ordie teeth to crab alongthe piece and cut out of pitch. Itwill a so beseen that with such a die the thread groove of an external Dardeletthread may be easily cut, since it is cut to approximately full depth byfour successive thin cuts, and that the bottom of the groove or thethread root is then brought to finished conoidal form by another'thin'cut entirely across the groove, the side walls of the thread ribbeing generated by these cuts.

A die having practically all of the advantages of the lastabove-described die may be provided by using therein the slightlymodified set of chasers shown in Fig. 1'7 instead of the set of chasersshown in Fig. 16. The chasers 24, 24 24 and 24 of this modified set ofchasers are constructed and operate precisely like the correspondingchasers 24 24 24 and 24 shown in Fig. 16, with the exception that theinclined or beveled throat surfaces T of the latter chasers are omitted,and the modified chasers have abrupt leading edge faces E lyingperpendicular to the surfaces P and extending from the outer edges ofsurfaces P to the backs of the chasers.

As shown, the usual beveled or undercut clamp- I ing surface U providedon chasers of this type at the back of each chaser, is, in the modifiedchasers shown in Fig. 17, located the same distance from the leadinglongitudinal side edge of the chaser as in the case of the form ofchasers be interchanged in the same die head. By employing the modifiedchasers shown in Fig. ,17, the thread may be carried still closer in itsfinal form to an abrupt shoulder on the piece being threaded than withthe chasers shown in Fig. 16.

The present application for patent is a continuation of my applicationsfor patent Serial No. 415,742, filed December 21, .929, and Serial No.442,224, filed April 7, 1930.

What I claim is:

l. A chaser for thread cutting dies, having a surface provided withparallel thread grooves with a common base line, the rib between thefirst and second grooves from the leading edge of the chaser being lowerthan the following ribs and having steep side wallsand a crest surfacelying parallel with the base line of the grooves,

of that groove which is nearest the leading edge of the chaser andcrossing the base line of the grooves as it approaches said chaser edge,and said following ribs each having steep side walls extending to thebase line of the grooves and a crest surface extending from one sidewall to the other at a slight slope relatively to the base line, thedirection of slope of said crest surfaces of the following ribsrelatively to the base line of the grooves being opposite to thedirection of slope of the beveled surface portion of the chaser surfacelocated adjacent the leading edge of the chaser.

2. A chaser for thread cutting dies as claimed in claim 1 wherein theribs are parallel with the side edges of the chaser, and the chaser hasa relieved cutting face extending transversely of the grooves from theleading edge of the chaser to a point beyond at-least the leading one ofsaid following ribs, and the cutting edges of said relieved cutting faceis so disposed as to lie parallel withthe axis of rotation of the diewhen the.

chaser is set in cutting position in the die.

3, A thread cutting die having a helical series of thread groove cuttingportions including en tering throat cutting portions with cutting edgesinclined outwardly toward the leading face of the die, followed bycutting portions of greater height having thread root cutting edgesparallel with the axis of the die, followed by finishing cuttingportions of still greater height having thread root cutting edgesinclined in a direction opposite to I said first-mentioned cuttingedges;

4:. A rotary die for cutting external Dardelet screw threads having aplurality of cutting edges spaced around its axis and each extendinglongitudinally of the axis and having only two thread groove cuttingteeth therealong, all of the thread groove cutting teeth of said diehaving bases of the same width equidistant from the die axis and lyingin a common helix, and also having sides making angles with the die axisof equal amplitude, the adjacent sides of the two teeth along eachcutting edge being connected by a portion of said edge narrower thanthetooth bases and tops and parallel with the die axis, and the leadingteeth of said cutting edges increasing in height in the order of theirrecession from the leading end of the-die and having tops parallel withthe die axis, while the following teeth of said cutting edges have topsslightly inclined to the die axis and area! greatest height at the sidesthereof nearest the leading and of the die and all higher than thehighest one of said leading teeth.

5. A rotary die as claimed in claim 4, wherein the teeth with inclinedtops are of equal height and are higher at their shorter sides than thehighesttooth with top parallel with thedie axis.

6. A rotary die for cutting external Dardelet screw threads providedwith a plurality of cutter elements having inner ends extendinglongitudinally of and spaced. apart around the die axis, each of whichends is profiled to present a row of thread groove cutting teeth havinga base line parallel with the die axis and a pilot cutting edge at theleading end of said row of teeth parallel with the die axis with all thecutting teeth of the die lying in a common helix and the tooth bases andsaid pilot cutting edges all equidistant from the die axis at the frontfaces of said cutter element ends, and a plurality parallel with the dieaxis while the remaining of. the am cutting teeth in said helix nearestthe leading end of the die increasing in height in the orderof theirdistance from the leading end of said helix and having top cutting edgesgroove cutting teeth in said helix are of still greater height and havetop cutting edges sloping toward the die axis in the direction of theleading end of the die.

7. A die as claimed in claim 6, wherein each of said pilot cuttingedgesextends from the leading side of the leading tooth of a rowtoward theleading end of the die and all said edges terminate in a common planeperpendicular to the die axis.

8. A die as claimed in claim 6, wherein all the teeth with sloping topcutting edges are of the same height and each cutter element has but asingle tooth with top cutting edge parallel with the die axis.

9. A multiple cutter element rotary die for cutting external Dardeletscrew threads each of the cutter elements of which has an inner end witha front face extending longitudinally of the die axis, said inner endsof said elements being spaced apart around the die axis and each havinga row of thread groove cutting teeth spaced apartat their bases and apilot portion at the leadingend of said row of teeth, all the groovecutting teeth of the die bring: in a common helix 105 and said ends ofsaid elements having pilot cut- -ti'ng edge portions-all parallel withand equidistant from the die axis at the front faces of said ends andall the interdental portions of said ends being parallel with andequidistant from the die axis at the front faces of said ends and thesame distance from the die axis as said pilot cutting edge portions, aplurality of the teeth in said helix nearest the leading end of the diehaving top cutting edges parallel with the die axis and being lower thanthe remaining teeth which have tops sloping toward the die axis in thedirection of the leading end of the die,.

10. A die as claimed in claim 9, wherein all the teeth have bases ofequal width and identical included angles between their sides whichconverge toward the tooth tops at equal angles to the die axis, and atleast one tooth on each cutter element has a top parallel with the dieaxis.

11. A multiple cutter element die for cutting 5 external Dardelet screwthreads, each of'the cut=- ter elements of which has-at its inner end arow of teeth spaced apart at their bases and a pilot portion at theleading end of said row, said cutter elements being arranged with theirinner 13o ends spaced apart around the die axis and extendinglongitudinally of the die axis and all the die teeth lying in a commonhelix with the bases of adjacent teeth of each cutter element connectedby an interdental portion, all the interdental portions and pilotportions lying parallel with and equidistant from the die axis, and aplurality of the teeth in said helix nearest the leading end of thehelix being lower than the other teeth and having tops parallel with thedie axis while the remaining teeth have tops sloping toward the die axisin the direction of the leading end of the die.

12. A die as claimed in claim 11, wherein all the teeth have the samebase width and all the interdental portions meet the same width, and,all the teeth have sides converging toward their tops at the same anglesto the die axis and are of the same height except the leading tooth ofeach row, the latter teeth increasing in height a having sidesconverging toward their tops withidentical included angles between thetooth sides, the tooth nearest the pilot surface being lower than theothers and having a top lying in a plane parallel with said commonplane, and all the other teeth being of equal height and having topsinclined to said plane with the higher sides of the teeth nearest thepilot portion.

14. A block-like chaser for cutting external Dardelet threads having afront end face, said chaser having parallel equally spaced rib-liketeeth and interdental and pilot surfaces at one face thereof allextending to said end face with the interdental surfaces and the pilotsurface adjacent the leading tooth lying in a common plane, the leadingtooth being lower than the others and having a top parallel with saidcommon plane while the other teeth have tops inclined to said plane withthe higher sides of the teeth nearest said leading tooth, all the teethhaving the same base width and sides converging at the same angle towardtheir tops, and said front face having a set back cutting portionextending from the leading side edge of the chaser and terminat ingbetween the second and third teeth from said side.

15. A chaser for thread cutting dies one surface of which is grooved toprovide a series of parallel thread ribs having a common base line andsepa rated by intervening parallel grooves, the rib between the firstand second grooves from the leading side edge of the chaser having acrest surface lying parallel with the common base line of the ribs andalso having steep side walls, and each 'ofthe ribs which follow saidlast-mentioned rib being higher than said rib and having two steep sidewalls which are connected by a crest surface that extends from wall to.wall with a slight slope toward said common base line, all of whichsloping crest surfaces incline in the same direction.

16. A thread cutting die having a helical series of successively actingthread cutting teeth around the die axis, which series of teeth includesall of the thread cutting teeth of the die and comprises one or moreteeth having a top cutting edge extending longitudinally of and parallelto the die axis, and one or more final cutting teeth extending closer tothedie axis than, and immediately following, the last-mentioned tooth orteeth and having a top cutting edge extending longitudinally of the dieaxis and inclining. toward the die axis in the direction of the leadingend of the die.

1'1. A thread cutting die having leading cutting teeth followed by'flnal cutting teeth which are provided with steep side cutting edgesand a thread root finishing top cutting edge connecting their sidecutting edges and inclining slightly toward the die axis in thedirection of the leading end of the die, said leading and final teethhaving their bases equidistant from the die axis and all being spacedaround the die axis in a common helical path of such small pitchrelatively to the thickness of the final cutting teeth that the latterteeth finish the out thread with a thickness exceeded by the width ofthe thread groove, each final cutting tooth having a profile largeenough to include the profile of o l-Ch one of saidv leading teeth, andeach leading tooth in at least one convolution of said helical pathimmediately in advance of the leading one of said final. cutting teethhaving side cutting edges and also having a top cutting edge whichextends parallel to the die axis.

18. A bar chaser of the tangential cutting type having a flat back cutaway along one longitudinal edge of the chaser to provide an undercutlongitudinal clamping shoulder for reception in a holder, and havingopposite said back a flat face grooved longitudinally to provide aplurality of parallel ribs whose bases lie in a common plane that isparallel to said flat back of the chaser, said ribs extending toopposite end faces of the chaser, one of which end faces is beveled fora portion only of the width of the chaser from the leading longitudinaledge of the chaser to provide a leading cutting end face portion and afollowing non-cutting end face portion, the leading rib or ribs only ofthose ribs which extend to the cutting end portion being lower than theremaining ribs extending to the cutting and non-cutting end faceportions, each of said remaining ribs having steep side faces connectedby a flat top face which is inclined relatively to the flat back of thechaser and slopes away from said flat back of the chaser in thedirection of the leading longitudinal edge of the chaser, and the lowerrib immediately in advance of said remaining ribs having a flat topsurface lying in a plane parallel to the plane of the back of thechaser.

.19. A chaser as claimed in claim 18, in which the, grooves between theparallel ribs have flat bottom walls connecting the bases of the ribs110 .and lying in a common plane parallel to the plane of the flat backof the chaser, and in which the cutting end face portion of the chaserhas a cutting edge lying in said plane and extending from the base ofthe leading rib toward the11'5 leading longitudinal edge of the chaser.

20. A thread cutting die having a plurality of serrated chasers, inwhich the cutting faces of all the cutting teeth provided by the chaserserrations are spaced around the die axis in a common helical path, inwhich path there is at least one cutting tooth on each chaser havingside cutting edges disposed steeply to the die axis and a top cuttingedgeparallel to the die axis, which teeth are immediately followed insaid path by one or more final cutting teeth having side cutting edgesdisposed steeply to the die axis and connected by atop cutting edgeinclined toward the die axis in the direction" of the leading end of thedie and extending closer to the die axis than the said top cutting edgesof the firstmentioned teeth.

21. A screw thread cutting die all of whose thread cutting teeth arearranged in a helical path around the die axis to cut sucessively,characterized in that the tooth which cuts behind all the others has atop cutting edge which slopes toward the die axis in the direction ofthe leading end of the die, and in that certain of the teeth which cutin advance of the aforesaid tooth have a top cutting edge which isdisplaced farther from the die axis than the top cutting edge of theaforesaid tooth and lies parallel to the die axis, the extent ofdisplacement from the die axis of the said top edges of the latter teethbeing difierent for each tooth.

22. A screw thread cutting die all of whose thread cutting teeth arearranged in a helical path around the die axis to cut successively,characterized in that the tooth which cuts bel5! hind all the others hasa top cutting edgewhich slopes toward the die axis in the direction ofthe leading end or the die, and in that certain 01 the teeth which cutin advance of the aforesaid tooth have a top cutting edge which isdisplaced farther from the die axis than the top cutting edge or theaforesaid tooth and lies parallel to the die axis, the extent ofdisplacement from the die axis of the said top edges of the latter teethbe- 10 1 ROBERT THOMSON.

